This invention relates to multilayer semiconductor structures, and, more particularly, to a solar cell with a phosphide-passivated germanium substrate.
A typical solar cell includes two or more layers of semiconductor materials. The materials are chosen such that, when light from the sun falls upon the solar cell, a voltage difference and current result between the layers. Electrical contacts affixed to the external surfaces of the top-most and bottom-most layers collect the current and provide external electrodes through which the useful electrical energy is conveyed.
Solar cells are used in a variety of terrestrial and space applications. In terrestrial applications, solar cells are often used in remote locations where it is uneconomical to provide power lines. In space applications, solar cells are used to power many types of satellites, such as communications satellites. The performance and operating efficiency of the solar cells is especially important in the space applications, because of the expense of lifting the solar cells and associated structure to orbit.
One well-known type of solar cell uses a homojunction germanium solar cell as the substrate upon which is deposited a gallium arsenide second solar cell. The homojunction within the germanium substrate typically is made by doping the surface region of the germanium substrate with arsenic. The gallium arsenide second solar cell is deposited overlying the doped region, to passivate the germanium and to form the solar cell. The efficiency of such a solar cell may be further improved by adding additional junctions, as by depositing a gallium indium phosphide solar cell overlying the gallium arsenide solar cell. The gallium indium phosphide solar cell converts the shorter wavelengths of solar energy to electrical energy more efficiently. The longer wavelengths pass through the gallium indium phosphide solar cell to the underlying solar cells, where they are converted to electrical energy.
The gallium arsenide/germanium and the gallium indium phosphide/gallium arsenide/germanium multijunction solar cells both are operable and achieve good results, but limitations remain. The inventors have recognized that better electrical characteristics could be obtained through better passivation of the germanium surface and shallower doping of the germanium. The present invention fulfills the need for improved electrical properties of solar cells, and particularly multijunction solar cells.
The present invention provides a multilayer semiconductor structure. The structure of the invention is useful as an independent solar cell or as the substrate upon which more complex multifunction solar cells are fabricated. The multilayer semiconductor structure is built upon a doped germanium substrate. The approach of the invention allows for better passivation of the germanium homojunction substrate and shallower doping profiles with better control over diffused dopant concentrations. The result is improved electrical characteristics, as compared with existing solar cells. The approach of the invention also allows for better heteroepitaxial nucleation of the overlying layers of added structure on the germanium substrate.
In accordance with the invention, a multilayer (i.e., two or more layers) semiconductor structure comprises a germanium substrate having a first surface. The germanium substrate comprises two regions, a bulk germanium region, and a phosphorus-doped germanium region adjacent to the first surface. A layer of a phosphide material overlies and contacts the first surface of the germanium substrate. Preferably, the phosphide material is gallium indium phosphide, aluminum indium phosphide, or gallium aluminum indium phosphide. In a preferred embodiment, a layer of n-type gallium arsenide overlies and contacts the layer of the phosphide material. Electrical contacts may be applied to the phosphide layer or, where present, the n-type gallium arsenide layer, and to the opposite face of the germanium substrate.
The bulk germanium region is preferably p-type germanium, and the phosphorus-doped germanium region is preferably n-type germanium. The phosphorus-doped germanium region in conjunction with the bulk germanium region form a homojunction, as required for the solar cell application. The overlying phosphide layer passivates the homojunction, and specifically the phosphorus-doped germanium region.
More complex multilayer structures may be deposited on top of the basic single-junction and multijunction solar cells described above.